Advertisement

Advertisement

Lucky strike in search for Earth’s most common mineral

By Colin Barras

Meteoric mineral: Percy Bridgman would be proud

(Image: Chi Ma/CALTECH)

Scientific accolades don’t get much bigger. Percy Bridgman, an American physicist active during the first half of the 20th century, has just had one-third of the planet named after him, although it’s a chunk of Earth that we will probably never see.

Earth’s lower mantle is largely composed of magnesium iron silicate, in the form of a mineral with a perovskite crystal structure. Given that the lower mantle is about 2000 kilometres thick, this mineral makes up 38 per cent of Earth’s entire volume, so it is easily our planet’s most common mineral.

It is surprisingly rare at Earth’s surface, though. So rare, in fact, that geologists have struggled to find a natural sample. And the rulebook is strict&colon; a mineral can’t be formally named with no natural sample to describe it.

Advertisement

Now, Oliver Tschauner at the University of Nevada in Las Vegas and his colleagues have finally discovered a sample that meets the criteria, meaning the mineral is nameless no longer. It will now be known as bridgmanite.

Crucial sample

“I would like to give professor Tschauner and his colleagues my congratulations,” says Masaaki Miyahara at Hiroshima University in Japan, one of the many geologists who has previously hunted for the crucial sample. Finally, he says, we can complete the naming of the mineral.

The bridgmanite that Tschauner and his colleagues found doesn’t come from Earth’s interior, though. It actually arrived on Earth in 1879, buried within a meteorite that slammed into Australia. The Tenham meteorite experienced temperatures of 2000°C and pressures of 24 gigapascals during its journey through the solar system – extreme enough to replicate conditions deep inside the Earth and allow bridgmanite to form.

The mineral should have broken down as the space rock returned to ambient temperature and pressure. The fact that it didn’t implies that the drop in temperature and pressure must have been extremely rapid, essentially “freezing” the bridgmanite in place before it could decay. Today it exists in certain regions of the meteorite as “crystallites” between 40 and 200 nanometres long, says Tschauner.

It’s important to be able to name bridgmanite finally, he says, but having natural samples of the mineral are significant for another reason. They can now be chemically analysed to reveal some of the trace elements that can naturally slot into bridgmanite’s crystal structure – which will help refine models of how the deep mantle behaves.

Mantle deep

For too long, says Tschauner, geologists and physicists have relied on a crude reductionist approach to understanding conditions deep inside the Earth. Because we know so little about the deep mantle, theoretical models usually only consider the abundant elements – magnesium, silicon and oxygen – that we know must be present. “You can’t just ignore the rest – it doesn’t work,” says Tschauner.

For instance, earlier this year Graham Pearson at the University of Alberta in Edmonton, Canada, and his colleagues discovered samples of ringwoodite, another mantle mineral, in diamond from deep within the mantle. The minor elements locked inside the ringwoodite provided good evidence for a massive “ocean” of water within Earth’s interior. Without the actual sample, we wouldn’t have known about the ocean.

Pearson says discovering the first natural samples of bridgmanite in a meteorite is important. “But it’s not the same as finding the mineral in a terrestrial sample,” he cautions. “Just as ringwoodite had been found for the first time in nature in a meteorite, its identification did not foretell the importance of water in the deep Earth.”

In other words, the search for bridgmanite from Earth’s mantle will continue, and there is no knowing what secrets it might reveal about Earth’s interior.

And who was Percy Bridgman? A Nobel laureate, Bridgman is sometimes called the father of high-pressure experiments. “His advances are what led to the ability to synthesise and study deep Earth materials,” says Thomas Sharp at Arizona State University in Tempe. “He made mineral physics possible.”